Hot melt pressure sensitive compositions are used widely in the adhesive industry due to their ability to adhere to a wide variety of substrates. Block copolymers are a preferred base due to their good heat stability, high adhesive strength, elastomeric properties, and compatibility with a wide range of tackifiers and plasticizers. A primary property of a hot melt pressure sensitive composition is that it remains tacky at room temperature and can be adhered to substrates when it has completely cooled from its molten state.
Applications requiring this property include labeling. Paper, polyethylene, polypropylene, polystyrene, composites or laminates, or other such substrates may be coated with the hot melt pressure sensitive adhesive, which is then covered with release liner, and then shipped for later application of the labels to substrates such as various containers, books, magazines, corrugated boxes, and envelopes to name just a few.
In addition to the types of labeling mentioned above, there is also bottle and container labeling where the hot melt adhesive is applied to the bottle or container itself, and the label is then adhered to such bottle or container on-line. The labels are not precoated and shipped as in the examples above. The most widely used materials for bottles or containers include, but are not limited to, glass, polyethylene terephthalate or "PET", polyethylene, polypropylene, and polyvinyl chloride (PVC). Such materials are generally difficult to bond, and the use of block copolymers as the adhesive base will facilitate the bonding.
Hot melts for bottle labeling are generally characterized by having low viscosities and long open times. Low viscosities are required because of the types of equipment used for bottle labeling and the restrictions that this equipment imposes. The equipment is run at very high speeds and stringing of the adhesive will occur if the viscosities are too high. Line speeds can be as high as 1000 bottles/minute.
These adhesives are further characterized by having good cold temperature flexibility, and excellent adhesion to a wide variety of substrates. Good cold temperature flexibility and excellent bondability are required because the bottles or containers are often for beverages, and are filled on-line prior to labeling. These containers are therefore often cold and wet which puts further demands on the adhesive. Bottle expansion which occurs during filling requires that the adhesive have good flexibility.
In addition, these adhesives require good heat resistance for shipping and for storage in the summer months. Temperatures inside shipping vehicles can exceed 50.degree. C. It can be difficult to achieve high heat resistance while maintaining good cold temperature flexibility, long open times, and very low viscosities because these characteristics work in opposition to each other.
A recent trend for bottle labeling manufacturers is to reduce costs by using lower gauge substrates. These substrates are often polyethylene, polypropylene, or composites of such materials which are heat sensitive. As the gauge of the film decreases, the films become more heat sensitive. Adhesive application temperatures must also be decreased, and optimum temperatures for these films are between about 120.degree. C. and about 150.degree. C. Conventional hot melt adhesives are applied at temperatures of about 155.degree. C. to about 175.degree. C. Viscosities of these conventional hot melt adhesives would be optimal, therefore, at temperatures of greater than about 155.degree. C. These hot melts could therefore not be applied at temperatures of less than about 150.degree. C. without machining difficulties such as stringing of the hot melt adhesive and nozzle clogging. As temperatures are decreased, it is also more difficult to get good adhesion to difficult substrates. It is, therefore, an advance in the art to offer hot melt adhesives that can be applied at lower temperatures while maintaining the characteristics of conventional hot melt adhesives such as good adhesion and high heat resistance.
U.S. Pat. No. 5,292,819 to Diehl et al., issued Mar. 8, 1994, discloses the use of a radial styrene-isoprene-styrene (S--I--S) block copolymer having the general configuration (pS--pI--pB).sub.n X where n is a number greater than 2 for use in various hot melt applications such as for disposable articles. However, Diehl does not teach the use of these polymers in hot melt adhesives for labeling applications. The adhesive formulations that Diehl teaches could not in fact be used for labeling applications because of the viscosity constraints imposed by hot melt labeling. The formulations of this invention have higher polymer contents and are too high in viscosity.
U.S. Pat. No. 5,399,627 to Diehl et al., issued Mar. 21, 1995, discloses the use of a radial S--I--S block copolymer having the general configuration (pS--pI--pB).sub.n X where n is a number greater than 2, for use in hot melt adhesives for tapes. The physical characteristics for a hot melt tape adhesive are vastly different than the physical characteristics of a hot melt labeling adhesive. Diehl teaches hot melt adhesives that would be too high in viscosity for application temperatures of about 120.degree. C. to about 150.degree. C. These adhesives would also be much too high in viscosity for running on high speed labeling equipment.
Similarly, U.S. Pat. No. 4,411,954 to Butch, III et al., issued Oct. 25, 1983, discloses a styrene-isoprene-styrene based hot melt pressure sensitive hot melt adhesive for diaper tape closures. These adhesives again would be much too high in viscosity to be run on high speed labeling equipment. The viscosities of this invention are shown to be greater than about 6000 cPs at about 163.degree. C. in contrast to the adhesives of the present invention where the viscosity is less than about 3000 cPs at 120.degree. C.
U.S. Pat. No. 5,057,571 to Malcolm et al., issued Oct. 15, 1991, discloses the use of a radial block copolymer having the general configuration (A--B).sub.n1 --Y--(B).sub.n2 where the A block comprises styrene and the B block comprises butadiene, isoprene, or mixtures thereof for disposable article construction. Malcolm does not teach the use of these products for labeling applications. Malcolm teaches hot melt compositions that would be much too high in viscosity for labeling applications. The adhesive compositions of Malcolm et al. disclose adhesive compositions that are greater than about 6,000 cPs at about 135.degree. C. in contrast to compositions of the present invention which have viscosities of less than about 3,000 cPs at about 120.degree. C. Therefore, the compositions of Malcolm et al. could not be used for applications where the requirements are for very low viscosities such as for bottle labeling.
U.S. Pat. No. 4,619,851 to Sasaki et al., issued Oct. 28, 1986, teaches the use of a combination of a thermoplastic rubber and an aliphatic hydrocarbon resin to achieve a slight incompatibility of the hot melt adhesive to improve die cuttability of labels. The rubber content of this invention is higher than the present invention. These products would be too high in viscosity for use on high speed labeling equipment.
U.S. Pat. No. 4,944,994 to Flanagan, issued Jul. 31, 1990, teaches hot melt pressure sensitive adhesive compositions for use in casemaking for bookbinding. This invention utilizes a radial styrene-butadiene-styrene block copolymer with a styrene content greater than about 35%. This invention does not disclose the use of these products for labeling. These products have high tensile strengths and high modulus which are advantageous for casemaking but would be disadvantageous for labeling applications. In addition, these products would not achieve the high tack properties useful for permanent labeling as in the present invention.
U.S. Pat. No. 4,714,749 to Hughes et al., issued Dec. 22, 1987, discloses the use of a fully saturated petroleum resin in combination with a saturated midblock thermoplastic elastomer such as Kraton.TM. G. Kraton.TM. G is a saturated, block copolymer having ethylene-butadiene as the midblock in contrast to the base polymers of the present invention which contain isoprene as the midblock. These adhesives are oil free systems, and contain saturated midblock elastomers in concentrations of greater than about 35% by weight of the total adhesive in contrast to the present invention where the polymer content is from about 5% to about 15% by weight of the adhesive. Hughes et al. does not teach how to make adhesives with very low polymer content. The compositions of Hughes et al. would be much higher in viscosity and not useful for applications where the compositions of the present invention are used.
U.S. Pat. No. 4,460,364 to Chen et al., issued Jul. 17, 1984, teaches the use of rubbery block copolymers as the base for removable pressure sensitive adhesives for sanitary products. Removable PSAs are precoated onto such products as band aids, labels, and pantiliners to name just a few. The PSA is then covered with a release liner to allow for shipping and storage of the finished product. When the finished product is used, the release liner is removed, the product is placed on the desired surface, (person, garment, machine, article, to name a few) and hand pressure is used to adhere the two substrates together. The adhesive, therefore, bonds at ambient temperatures, without heat. This is not a permanent bond, however, and upon removal of the label, band aid, pantiliner, etc., it is desired that no residue should be left on the remaining surface. The desired characteristics of removable PSAs therefore, include aggressive quick stick, which is the ability to immediately adhere to a substrate upon application, lack of residue on the remaining surface, and low peel adhesion. The adhesives of the present invention, in contrast, are permanent grade PSAs used to permanently bond two substrates together rather than forming a temporary bond. They are used to mate two substrates together using application temperatures of greater than about 120.degree. C. but less than about 150.degree. C., rather than at ambient temperatures. The adhesive compositions of Chen et al., have high block copolymer contents, greater than about 18%, and the viscosities would also be much too high in viscosity for high speed labeling equipment. The adhesives of Chen et al., would be applied using such methods as slot die coating or spray application.
U.S. Pat. No. 4,136,699 to Collins et al., issued Jan. 30, 1979, discloses a disposable article for absorbing fluids comprising a hot melt adhesive which contains from about 10% to about 20% by weight of a radial block copolymer having the general configuration, A--B--A, where A is polystyrene, and B is an essentially saturated poly(mono-olefin). An example of the midblock would be ethylene-butadiene. These adhesives are also removable PSAs. This is in contrast to the present invention where the B block is isoprene, and the adhesive is a permanent grade PSA.
U.S. Pat. No. 5,360,854 to Bozich, Jr., issued Nov. 1, 1994, discloses a hot melt pressure sensitive adhesive based on styrene-butadiene-styrene block copolymers, such adhesives having high block copolymer contents. In addition, the block copolymer of Bozich contains percent styrene from about 35% to about 55% by weight of the block copolymer. Higher percentages of styrene result in hard block copolymers that are difficult to tackify. These adhesives are also oil free and are high in viscosity. They could not be used in applications where those of the present invention are used.
The present inventors have found that using low amounts of radial S--I--S block copolymers, having the general configuration (pS--pI--pB).sub.n X or (pS--pI).sub.n X as the base polymer for hot melt pressure sensitive adhesives allows for low viscosity adhesives that can be applied at temperatures of about 120.degree. C. to about 150.degree. C. while still providing excellent tack, adhesion, flexibility, cohesive strength, and heat resistance.